The present application is directed to a thermally conductive electrically insulating connection between two members and the method of forming the connection.
In the manufacturing of power assemblies having at least one power component, a thermally conductive, electrically insulating connection is required between the power component and a heat sink or cooling body. This connection provides two functions one being the conducting away of lost heat and the other being ensuring a sufficient electrical insulation from the cooling body or heat sink. For the manufacture of connections of this type, it is known to attach the power assembly or an individual power component to a copper bearer or member for example by means of soldering or gluing. The copper bearer or member, which serves as a heat spreading element, is connected with an associated heat sink predominantly via an electrically insulating ceramic intermediate piece which is preferably made of aluminum oxide. The quality of the heat conductivity also depends in particular on whether this connection is made by mechanical pressing, gluing or soldering. To obtain a mechanical solidity in manufacturing and processing of this type of connection between a power component and cooling or heat sink, the ceramic intermediate pieces must be thicker than is required for the electrical insulation and this is a disadvantage. This increased thickness will limit the achievable heat conductivity of the connection.
European 0 042 693 discloses a ceramic intermediate piece which avoids the above described disadvantage and which is obtained by means of plasma-sprayed ceramic layers. The ceramic layers are thus attached to the heat spreading elements and the heat spreading elements are then glued to the associated cooling bodies via the ceramic layers. The layer consistency and layer thickness determines the heat conductivity and the electrical insulating effect and can be adapted to different requirements within broad limits. However, the hygroscopic property of the plasma sprayed ceramic layers have disadvantageous effects particularly when, for example, the construction of the connection cannot be shielded against air moisture using insulating gels, for example silicon based gels, in a long term stable manner.
A power assembly is disclosed in German Gebrauchsmuster 89 14 493 U1 that comprises a thermal conductive, electrically insulating connection between a bearer or member and the cooling body or heat sink. The insulating layer is thus fashioned not as a previously manufactured part but rather as a coating that is either applied to the cooling or heat sink and glued to the bearer or applied to the bearer and glued to the heat sink or cooling body. In a variant, an anodic coating is formed as an insulation layer on the surface of the cooling body which is made of aluminum. This anodic coating which is formed by anodic oxidation of the aluminum, does not however comprise a smooth aluminum oxide surface but rather a nonuniform surface in the manner of a cratered landscape. If the remaining thickness of the anodic coating between the crater valleys is too small with respect to the required electrical strength, the anodic coating is additionally coated with a layer of polytetrafluorethylene which is just thick enough to fill the craters created during the oxidation. The smoothing of the surface of the anodic coating makes the insulating layer thicker and increases the electrical strength.